With rapid development of mobile communications in a technology level, the demand for communications in the consumer market is also promoted rapidly. The communication technology has gradually changed from technology-oriented to market-oriented. Nowadays, the mobile communication technology is to enter the age of 5G communication in 2020, the consumer market has increasingly concrete demands for future 5G communications, including higher communication speeds, wider user coverage and more accessing users. As the Internet of things, the Internet of vehicles, the Internet of bodies and other various network concepts in future have been gradually accepted, large-scale multiple access will become a normal demand of the future market. However, at present, limited time frequency resources can only bear limited user access, and thus, for future large-scale user access, the study of an overload transmission system has certain necessity and urgency. In an overload system, multiple accessing users may use the same time frequency resource through competitive random access, and then data packets of multiple users may collide at a base station receiving end. At the same time, in a future networking technology, user access is sporadic, that is, the number of packets colliding at the base station receiving end per unit time depends on the excitation probability of the accessing users to a great extent and is generally less than the total number of the accessing users. Therefore, to detect and solve collision in a sporadic overload transmission system, a combination-based spreading overload access technology comes into being.
As a whole, the sporadic overload transmission system is one form of a competitive random access system. In the system, each user randomly activates and sends a data packet in different times, and thus the data packets may probably collide at the base station receiving end. For the overload system, the higher the overload rate is, or the higher the excitation probability of each user is, the higher the probability that data packets of different users collide at the base station receiving end is, and the more the number of the colliding data packets is. As shown in FIG. 1, a cell has a single base station BS and K access user equipments UE1, UE2, . . . , UEK. Herein, suppose that the number K of the user equipments is greater than the number of time frequency resources of the system to make the system an overload system. The K users randomly activate and send data packets at different time slots within a period of time, for example, the UE 2 sends data packets at time slots 3 and 4, the UE3 sends data packets at time slots 2, 4 and 6, while the UE1 does not send data packets in a current time period; in this way, at the time slot 4, the base station may receive a superposed signal of the two users UE2 and UE3, that is, collision of data packets occurs in UE2 and UE3 at the time slot 4.
In order to try to correctly decode the colliding packets without retransmission, it is very important to distinguish the user equipments from the colliding packets. For a traditional non-overload system, the receiving end may make distinction through orthogonal resources. For example, in a Code Division Multiple Access (CDMA) system, a transmitting end uses orthogonal spreading codes (referred to as “orthogonal spreading codes” herein, and irrelevant spreading codes) to spread the sent data, and the receiving end may distinguish different user equipments through the orthogonal spreading codes. However, for the overload system, the number of the orthogonal spreading codes is evidently limited for a great number of user equipments to be accessed.